Correlating Rashba Suppression with Exciton Binding in Low-Dimensional Metal Halide Perovskites

We used DFT-1/2+SOC band structures mapped to Wannier tight-binding Hamiltonians and solved with the Bethe−Salpeter equation to examine how confinement, polymorphism, and metal off-centering tune excitons in Pb- and Sn-iodide perovskites in 3D (MA(Pb or Sn)I$_3$), quasi-2D (BA$_2$(PborSn)I$_4$), and model quasi-1D (BA$_3$(PborSn)I$_5$). The 3D polymorphs show a clear trend: as symmetry increases from orthorhombic to tetragonal to pseudocubic, E$_b$ decreases. At fixed dimensionality, restoring local metal-centered symmetry suppresses Rashba splitting. It reduces or preserves E$_b$, indicating that SOC affects excitons mainly through band-edge structure rather than direct strengthening of the electron−hole Coulomb kernel. Low-dimensional compounds retain much larger E$_b$ than 3D analogs, although quasi-1D is not monotonic with quasi-2D. Photoluminescence spectra place the lowest excitons at the optical gap, consistent with bright ground-state excitons.